1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with an image forming apparatus such as a facsimile, a printer and the like and adapted to supply a sheet or an original to an image forming portion or an image reading portion.
2. Related Background Art
There has been proposed a sheet supplying apparatus in which a plurality of sheets are set and are successively supplied to an image forming portion, or a sheet is supplied to the image forming portion from an intermediate tray for temporarily holding the sheets. This is to form an image of a rear surface of the sheet having a front surface on which the image was already formed or originals are successively supplied to an image reading portion, as shown in FIGS. 15A and 15B. FIG. 15A shows a waiting condition of the sheet supplying apparatus, and FIG. 15B shows a condition that the sheets are being supplied.
In such a sheet supplying apparatus, the sheets stacked on an intermediate plate 503 are successively separated and supplied by a separation roller 501 and a separation pad 502 urged against the separation roller. In the waiting condition of the sheet supplying apparatus, as shown, the intermediate plate 503 is pushed downwardly by a cam (not shown) to separate it from the separation roller 501. Thus, the sheets can be set on the intermediate plate 503.
When a sheet supplying operation is started, the cam pushing the intermediate plate 503 downwardly is rotated by a clutch means (not shown) such as a solenoid, with the result that the cam is disengaged from the intermediate plate 503, thereby lifting the intermediate plate 503 by a spring 504. Consequently, an uppermost sheet on a sheet stack rested on the intermediate plate 503 is urged against the separation roller 501. When the separation roller 501 is rotated, the sheets are fed out. Then, the sheets are separated one by one between the separation roller and the separation pad 502 and the separated sheet is sent to a downstream image forming portion or a downstream image reading portion.
However, in the sheet supplying apparatus having such a construction, a drive source for driving the cam must be provided independently from a drive source for driving the separation roller. Or, when a common drive source for driving the separation roller and the cam is used, a drive switching mechanism such as a solenoid or an electromagnetic clutch for selectively switching connections so that a driving force from the drive source can be transmitted only when it is requested must be provided. Thus, the apparatus is made complicated or is expensive.
In order to solve such a problem, there has been proposed a sheet supplying apparatus as disclosed in U.S. Pat. No. 5,219,155. Such a sheet supplying apparatus will be explained with reference to FIGS. 16A through 17. FIG. 16A shows a condition that, in a normal rotation of a motor (not shown), when a cam 204 is rotated in a direction shown by the arrow D to release regulation of an intermediate plate 202, the intermediate plate 202 is pushed upwardly by a pressure spring 212 to a sheet supply permitting position, and FIG. 16B shows a condition that, by rotating the motor in a reverse direction to rotate the cam 204 in a direction shown by the arrow C, the intermediate plate 202 is pushed downwardly in opposition to the pressure spring 212 to a waiting position.
A separation roller 200 is connected to the reversible motor so that, by rotating the motor, the cam 204 is driven to lift or lower the intermediate plate 202 on which a sheet stack is rested. A driving force of the motor is transmitted from a drive shaft 200a of the separation roller 200 to a clutch 210 (FIG. 17) through gears 206 and 208. The cam 204 is connected to the clutch 210 to be appropriately controlled by it.
The clutch 210 has a construction as shown in FIG. 17 so that, when the motor is rotated in the normal direction, a clutch spring 214 is loosened not to transmit the driving force to the cam 204, and, when the motor is rotated in the reverse direction, the driving force is transmitted to the cam 204. Further, the cam 204 is urged against a side surface of the gear 208 by an urging spring 216.
With this arrangement, during the sheet supply, when the motor is rotated in the normal direction, the separation roller 200 is rotated and the cam 204 is rotated in the direction D through the clutch 210, with the result that the intermediate plate 202 is lifted to urge the sheet stack against the separation roller 200. In this case, since the clutch spring 214 of the clutch 210 is loosened, the driving force is not transmitted by the clutch spring 214, but, the cam 204 is rotated due to a friction force between the cam and the gear 208.
After the sheet supplying operation is finished, the motor is rotated in the reverse direction to rotate the cam 204 in the direction C (FIGS. 16A and 16B) through the clutch 210, to thereby lower the intermediate plate 202. When the intermediate plate 202 is completely lowered by the cam 204, this condition is detected by a sensor (not shown) to stop the motor. In this way, by utilizing the normal and reverse rotations of the motor for driving the separation roller 200, the lifting/lowering of the intermediate plate 202 can be controlled.
However, in the sheet supplying apparatus having such a construction, when the intermediate plate 202 is lifted, since the cam 204 is rotated by utilizing the friction force between the cam 204 and the gear 208, the intermediate plate 202 cannot be lifted and lowered, if the friction force is small, which causes the poor sheet supply.